Mod Con Boiler vs Heat Pump – Nassau County, Long Island

[_Jerry_]

Hi all,

I am a new homeowner with minimal knowledge of heating systems prior to coming across this forum last week. Thanks for all the great info. It has been tremendously helpful.

I am looking to replace my existing gas boiler with a mod con unit. I may or may not have to replace my existing indirect DHW tank (will take advice). I contacted several contractors, all provided different (over)sizing options and wide range of costs. I would like further guidance in choosing the right equipment. Any input will be much appreciated. Background:

Location: Nassau County, Long Island, NY 11791

House: Two-story split level with “half” basement, built in 1955. Energy audit was performed after moving in last month. Blower door test measured about 5000 CFM. The house is generally poorly insulated and very leaky (for instance, there is an existing 3’ x 3.5’ whole house fan vent at the ceiling of the second floor directly open to attic). Before December, at the minimum, I am looking to air seal (minimum 25% CFM reduction), and insulate attic floor. I plan to eventually air seal basement rim joists as well. Gas boiler and DHW tank are located in basement. Basement has minimal insulation.

Existing Gas Boiler: 1979 cast iron Hydrotherm HC-165 (most likely from 1980s). Input 165,000 BTU/hr. Output 130,000 BTU/hr. Energy audit company measured combustion efficiency at about 70%.

Existing DHW Tank: Appears less than 10 years old. Techtanium TT-40 Indirect Water Heater. 40 gal.

Existing Heating Zones: Two Zones – ¾” copper pipe:

Zone 1
First Floor – 29’ total (23’ of slant fin in dining room and kitchen; remaining 6’ of fin are about 6” wide enclosed in old metal crate/vent)
Second Floor – 18’ total (7’ of slant fin in bathrooms; 11’ of the older/wider fin in three bedrooms combined)
Attached Garage (uninsulated) – 11’ slant fin (recently added by previous owner)

Zone 2
Den – 10’ slant fin

Total fin length = 68’
It will be about 57’ after removing the garage loop.

Heated Areas:

Zone 1 First Floor = 580 ft2
Zone 1 Second Floor = 780 ft2
Zone 2 Den = 475 ft2
Total = 1835 ft2

Other Areas With No Heat:

Attached Garage = 380 ft2
Basement = 580 ft2

I will start with some basic observations/questions. Please feel free to comment:

1) Heating Load - Without running Manual J and past gas usage data (no access), I am not sure if I can accurately estimate load. Also old usage data may not be relevant since it does not reflect benefits of new insulation. For heating range from +15F to +65F (design), it appears heating load of 30,000 BTU/hr is a conservative starting point. A more realistic range after air seal may be in the 20,000 BTU/hr range? Median temp last winter was about 40F, so seasonal heating load is likely on the order of 10,000 to 15,000 BTU/hr?

At 1.4x oversizing of design, its still between 28,000 BTU/hr and 42,000 BTU/hr. Certainly not 80,000 BTU/hr, or even 160,000 BTU/hr that contractors suggested to me so far.

2) Minimum Fire Rate – The popular mod con choice here seems to be the HTP UFT-80W at 8000 BTU/hr minimum fire input. Operating at 120F AWT and 57’ of baseboard, they will emit about 57’ x 200 BTU/hr per ft of fin = 11,400 BTU/hr.

But how can I confirm output of the old 6” wide fin?

If I have two equal zones, it appears I will need minimum 40' fin (assuming 200 BTU/hr per ft at 120F AWT) per zone to avoid short cycling in condensing mode with HTP? How do you factor in the usage of the indirect DHW? HTP UFT-80W seems to be a good choice. However, finding an experienced local HTP installer is a bit of struggle.

3) What are the deciding factors as to whether or not to replace or keep the existing indirect DHW tank? If I have to replace the tank, which model/feature will be a good fit with the mod con?

4) Side question. Since its only function is to heat the indirect DHW as needed, how efficient do mod con boilers operate during the summer months with indirect DHW setup?

Just some basic questions for now. Just want to make sure I am on the right track. Any input will be appreciated.

Thanks,
Jerry

 

[BadgerBoilerMN]

You have done a good job. All you are lacking is a room-by-room Manual 'J' to determine the highest per sq.ft. load and balance controls if necessary.

If it is hard to find an installer for the UFT imagine finding a service company on a cold holiday weekend. If you have it serviced annually you may not need much of that.

I am currently designing a deep energy retrofit in Brooklyn with similar loads and offered the UFT along with the lower minimum output of the Laars Mascot and the Dunkirk Helix pending my interviews with the local reps, wholesalers and potential installation/service companies since it is all about proper installation and regular informed service after the proper equipment is secured.

The indirect specs can be considered with the new lower output. If you go with the smaller units a bump in indirect capacity may be in order.

 

[Sponsor]

 

[Dana]

1> Running a fuel use heat load calc on the "before" picture is still useful, since it establishes a firm upper bound. You could also run a room-by-room I=B=R method load calculation using a spreadsheet tool, if you can estimate the various U-factors of the exterior assemblies.


2> When you say 6" wide fin, are you talking about finned convectors in a taller cabinet, or fat baseboards or something else ?? The height of the convector cabinet, the spacing of the fins, and the total profile of the fins (height & width) all affect the output.

vs.

vs.

If you can find a similar finned convector to what you have from some current manufacturer, it would be a starting point.

A typical old school 30- 32" tall convector with 6" fins will put out about 4000BTU/hr per foot of cabinet width with 215F steam, and something like 800BTUH/ft with 120F water. If it's only 18" tall it'll be more like 700 BTUH per foot @ 120F. Here is the tech manual from one such vendor. Here is an output chart for some modern 4.25" convectors. With a better description we can probably find a similar product out there.

The minimum-fire output of the UFT-080W is about 7600 BTU/hr, so with typical ~2" fin tube baseboard and 120F AWT, at 200BTU/hr per foot it takes 7600/200= 38' to balance perfectly, but in practice with a couple of zone operating you'll probably do fine with as little as 30', since the run times will be long, and zone calls will tend to overlap.

3> The age and condition of the indirect water heater matters, as does it's volume relative to the biggest tub you have to fill. If it's less than 10 years old and seems to be working OK, and is big enough to fill a tub it's probably worth keeping. If it was installed prior to the first Clinton administration, maybe not. There is nothing peculiar to a mod-con that affects the decision of which indirect gets installed- it's really a matter of volume. If you have a master bath shower with six 2 gpm side sprays and love to take 20 minute showers you need to do the math on the total volume it takes.

4> The summertime efficiency of the mod-con in hot-water only mode depends on the storage temperature in the tank (lower=better for efficiency, but below 120F is the high-legionella growth zone), the volume/duty-cycle, and how well the plumbing between the boiler & indirect is insulated, etc. If you look at the testing details in Appendix 11, a ridiculously oversized 160KBTU 95% efficiency boiler (with a 56K minimum output) with a 40 gallon indirect operates at about 59% efficiency in water-heating-only mode using interpolations of their regression curves. But a well installed 80K condensing boiler with a min-fire output of 7600 BTU/hr with insulating plumbing should beat that by quite a bit. It's not quite as efficient as best-in-class condensing water heaters, but usually way more efficient than a center flue standalone.

 

[_Jerry_]

BadgerBoiler,

I have contacted a local supplier as NY_Rob suggested in his post, who can place an order for me, be shipped in a month, but does not stock any replacement parts. That makes an experienced contractor even more critical, not just for installation, but future servicing and repair. I will contact HTP directly tomorrow for some local support. If you have a list of qualified contractors in the NYC/Long Island area and do not mind sharing, I would love to take a look. What are some important questions to ask them as a check for their experience with HTP product?

Dana,

Thanks for clarifying. They are tall fin-tubed convector, similar to the attached top photo from one of your older posts. The bottom photo shows the inside of the cabinet at my house.



Based on your reply above, my very-rough estimated output for each of my 3' convector ~ say 700 BTU/hr x 3' ~ 2100 BTU/hr at 120F, or the equivalent of 10.5' of slant fin at 120F?

It will be very helpful if you can estimate with the info below:

Cabinet Size – 3.3’ length x 0.5’ wide x 2’ high
Length of Fin Section – 3.05’
Number of Fins Counted – About 70 fins per foot of fin section
Fin Dimension: 0.5’ wide x 0.2’ high

Thanks so much in advance.

Jerry

 

[Dana]

The 33" wide 6" deep 24" tall convector should put out somewhere between 1800-2400BTU/hr @ 120F AWT depending on the amount of dust-kittens & bent fins, etc. Call it ~2000 BTU/hr give or take, so yes, it's comparable to ~10' of SlantFin baseboard.

Can't help you on the contractor hunt front, but HTP should be able to find you one.

 

[_Jerry_]

Thank you, Dana.

In addition to heating, I am also looking to add insulation and replace existing central AC. I came across your replies elsewhere related to both last night. I would love your inputs particularly on heat pumps. One of your comments really struck a chord with me, in that the time to replace any equipment is also the time to review all aspects of the system.

I briefly described my existing insulation in my OP. Below has more on my existing central AC.

Condenser Unit Outside: 2015 Rheem, SEER 13 (replaced by previous owner just before closing last month)

Air Handler Unit in Attic: 25+ years. Should be an R-22 system. Observed corroded coils and water in pan during home inspection in June. Very minimal was done since.

Ducts are located in attic, generally very leaky. I added mastic tapes the best I could since moved in. I received quotes on different AC replacement options, a) replacing air handler with same SEER 13 rating as condenser, or b) replacing both units with higher SEER. All contractors randomly proposed 3-ton, based on no calculation.

My original thoughts were that a) 3-ton is likely over-sized, b) still need to address leaky ducts in attic, and c) address insulation and heating first, then cooling because winter is around the corner. I allocated budget of about $15,000 for all three upgrades. After reviewing preliminary quotes, I figure about $5k to $7k heating, $3k to 7k cooling, remaining goes to insulation. If budget doesn’t allow, I will skip on some insulation for later.

After briefly reading about heat pumps, it appears my house can be a good fit based on estimated heating and cooling loads (conservatively 20k to 30k BTU/hr at +15F post-insulation; 2-ton using VERY general guideline you posted elsewhere). My thoughts/questions:

1. How does heat pump compare to the conventional heat/cooling option (condensing boiler+central AC) in terms of cost (upfront installation+future servicing), efficiency, reliability, and comfort? We cant predict future unit cost of kWh vs therm, so assume both are equal. Mainly trying to see if $15k will be enough for everything.

2. I should prioritize insulation ahead of everything to further reduce heating/cooling loads, because this affects sizing. I will run I=B=R as suggested for post-insulation condition. I have asked local gas company and previous owner for past usage. No luck. Perhaps I should delay any replacement until next spring to allow post-insulation data collection. Also it will be expensive to completely insulate existing finished basement. Maybe just air seal rim joists. If I keep basement unheated/uninsulated, how much does it affect sizing and comfort?

3. Can you provide a few heat pump manufacturer/model/output that fits my house? Any recommendation/personal experience you can share?

4. Any additional distribution system equipment/retrofit to the house associated with heat pump? I am guessing I will need to replace my indirect DHW tank with a direct gas-heated one? Do both AC and heat use duct (that means no need for baseboard)? What option is available if I want to eliminate any duct in attic?

5. Anything else I need to consider? Manual J? Sure, will need it for local rebates anyway. But after reading some of your posts, I=B=R appears accurate enough for sizing?

Thanks,
Jerry

 

[Dana]

A 3 ton AC unit is definitely oversized for ANY ~1800' house, unless you accept the additional load of the duct leakage outside the thermal envelope as an OK situation. The real cooling load for the house (not counting the parasitic load) is more likely to be in the 1.5 ton range, maybe as high as 2 tons if you have a lot of west facing glass.

Depending on the layout of the second floor, you can probably do what BadgerBoilerMN does for cooling: Install a 1.5 ton mini-split at the top of the stairs and leave the doors to the upstairs rooms open, letting convection cool the lower portion. A floor plan for both floors would be useful for determining whether a ductless heat pump solution would work for you , and how may heads/cassettes, and what type.

A 2 ton or 2.5 ton 3-zone multi-split will almost certainly cover your heating load, and would be a bit oversized for your cooling load, but with sufficient modulation range and right-sized heads/cassettes would still hit it's SEER numbers. In my neighborhood something like the 2-ton Fujitsu AOU24RLXFZ would run about $7-8K all-in, professionally installed in competitive bidding. It's good for over 22,000 BTU/hr @ +17F outdoors in a 70F indoor temp, or +15F (your 99% outside design temp) with a +68F (code min) indoor temp. The 2.5 ton Mitsubishi MXZ-3C30NAHZ puts out over 28,000 BTU/hr even down at +5F, and should come in under $9K in competitive bidding.

If going fully ducted with bigger air handler it's important to bring the ducts inside, which is complicated unless you insulate the attic at the roof deck, which is a fairly expensive proposition. The first floor could be probably be served fine with a 1-ton or 1.5 ton Fujistu 12RLFCD or 18RLFCD mini-ducted unit mounted under the basement ceiling with the ducts running in the basement or insulated crawlspace, depending on what you meant by "half basement", or a Mitsubishi MVC series air handler. The Mitsubishi MVC series has better blower capacity than the Fujitsu mini-duct cassettes, but a much lower modulation range, which makes sizing it more critical. Costs for the ducted solutions are all over the place, and not all mini-split contractors are comfortable installing them.

For more traditional (but still highly efficient modulating) ducted systems, you can probably heat/cool the place reasonably efficiently with a 2-ton (smallest of the line) Carrier Greenspeed, but that has less modulation range than the MVCs. You can play around with the different air handler & compressor capacities for the Greenspeed using this online tool.

From an operating cost point of view we'd have to look at your all-in delivered gas & electricity rates (the total bill, divided by therms, CCF, or kwh.) Long Island has some of the most expensive electricity & gas in the lower 48, and using the statewide NY average rates would be pretty far from reality.

 

[_Jerry_]

Dana,

Thank you again.

1. Fuel-Use-Based Heat Load - The previous owner was nice enough to share their old ACTUAL gas usage between November 2015 and March 2016. The fuel-use-based 99% heat load assuming +15F outside design temp ranged from 37,000 to 42,000 BTU/hr. Including 1.4x oversizing, it’s 52,000 to 59,000 BTU/hr. Calculations and usage data are included in snapshot below.

2. I=B=R Heat Load at Existing Pre-Insulation Conditions - See calculation/assumptions in snapshot below. Coincidentally, I calculated to about 40,000 BTU/hr at existing conditions (pre-insulation). I was fairly aggressive with U-factors. Please correct if there is anything wrong. I completely guessed on items highlighted in red.

3. I=B=R Heat Load at Post-Insulation Conditions – See calculation/assumptions in snapshot below. Insulation cuts heat load by about 40%, now at about 25,000 BTU/hr. I would love your inputs on insulation work in general. Does reduction make sense with the proposed insulation work? My goal is to add enough insulation so there is very small chance for even needing backup heat. It seems there is a lot of reduction available by insulating walls. But my family has already settled in the house, so cutting drywall to add insulation may not be best. Dense-packing cellulose is another option, but expensive.

4. Snapshot of floor plan sketch is included below. There are four levels in the split house, 2nd floor, 1st floor, den, and basement. Each level occupies roughly half of the building footprint. Please let me know if ductless heat pump is an option/how many cassettes/what type.

5. Just in case, does backup heating use electric or gas in the Fujitsu and Mitsubishi units? I am guessing gas heats better in frigid condition and cheaper?

6. My local electric offers rebates, but both Fujitsu and Mitsubishi units fall just short of their EER requirement. See snapshot of their requirements below. Are there other options?

7. Based on my last bill, local electric costs $0.21/kWH, while local gas costs $1.58/therm. Please let me know if we can roughly estimate operating cost in normal condition.

Sorry for the long post. I appreciate all your help.

Thanks,
Jerry

 

[Dana]

The fuel use calculations yielded a load of 37-42K, and the U-factors of the "before improvements" picture are reasonable, and the calculated 36.5K is in pretty good agreement with the fuel use load numbers. Given the age & oversizing factor of the boiler I'd be surprised if your true heat load before improvements isn't really closer to 35,000 BTU/hr. (Sanity-check: 35,000 BTU/hr into 2400' of space is 14.6 BTU/hr per square foot, near the middle of the distribution curve for 2 x 4 framed houses.)

If you air seal & insulate the exterior foundation walls of the basement, get rid of the holes in attic floor for the ducts & air handler making that air tight then seal up the flue, the stack-effect driven air infiltration will drop precipitously, and the load will probably drop to the 30-32K range, (maybe even drop under 30K).

So, do I understand this correctly that the den is below the kitchend/living/dining room area, and the upper floor is above the den & garage? Are the stairwells doored off, or it is open? (It makes a difference on how a heat pump or mini-split system might be configured.)

Fuel costs:

Gas: at 95% efficiency you get 95,000 BTU/hr of heat per therm into the house, so per million BTU (MMBTU) delivered that's 1M/0.095M= 10.5 therms/MMBTU, which at $1.58/therm is $16.60/MMBTU. To that some amount of power use for pumping & controls, etc. would need to be added.

A pretty-good 3-zone ductless multi-split will deliver ~10,000 BTU/kwh (that would be a as-used HSPF of 10.0), but separate mini-splits will usually be in the 11s or higher. A Fujitsu mini-duct unit will also be in the 11s. But for ease of calculation let's assume 10,000 BTU/kwh, so that would take 100kwh/MMBTU, which at a price of $0.21/kwh costs $21.00/MMBTU.

That's quite a bit more per MMBTU in heating, but the up front costs of having a single heating & cooling system will run about the same as just the condensing boiler installation. Also note, as PV solar continues on it's significant learning curve it's likely that you'd be able to offset all or most of your heating costs with rooftop solar at a steep discount from the 21 cents, provided you have reasonable shading factors. Right now the US average cost of small scale solar is running ~$3.50/watt, but there are some locations in the US where it's already down to $2-ish, and in Australia (which has a more mature & competitive market) it's running about USD $1.25/watt. That is a price point we should see in the northeastern US before 2025, maybe even by 2020 (in my dreams, anyway. )

On Long Island you'll get a capacity factor of at least 15% out of the panels (that's an average of 15% of the nameplate watts over the 8760 hours in a year). At $3.50/watt, 5% interest and a 15% capacity factor the levelized cost over 20 years of system lifecycle is about 21.5 cents. With the 30% federal income tax subsidy, even without other incentives such as solar reneble energy credits (SREC) etc. the cost comes to $2.45/watt, with a 20 year levelized cost @5% interest of 15.1 cent/kwh, which when run through a mini-split comes out to $15.10/MMBTU, which is marginally cheaper than natural gas.

If one waits until the price point of $1.25/watt arrives the levelized cost would be 6.4 cent/kwh, or $6.40/MMBTU when run through a mini-split at an HSPF of 10, but I would expect by that time net-metering at retail will have gone away, and your actual cost will be higher than that, but still cheaper than natural gas.

So...

...have you looked at your rooftop on Geostellar or Project Sunroof? If you follow that up, get quotes on purchased systems, not s0lar leases or third party ownership PPAs. (There are plenty of potential pitfalls for the latter.)

 

[Dana]

I forgot to mention- mini-splits and mini-duct cassettes to not have backup power, and have to be sized for the entirety of the 99% outside design load on heat pump alone by consulting the engineering manuals and extended capacity tables. But since the test conditions in an HSPF/SEER submittal for heating is a 70F indoors/+17 outdoors, it's the same temperature difference as a code-minimum +68F indoor temp and the +15F 99% outside design temperature. While the mini-split still has signifcant capacity at lower temperatures, it's possible to size them in your location using the "nominal" or "rated" heating capacity.

The Mitsubishi MVZ air handers do have a resistance heating backup option, which allows you to undersize the heat pump and still have a minimum guaranteed capacity at any outdoor temp.

If you broke up the ductless into separate mini-splits rather than a multi-split you can meet or exceed just about anybody's efficiency requirements.

1.5 ton mini-duct

half ton wall coil type

3/4 ton wall coil type , another 3/4 tonner

1.25 tonner, another one

If there is some confusion on the EER specification, talk to an installer and figure it out.

Separate mini-splits is often cheaper too, but you do end up having to figure out where to place multiple compressor units. It's not too bad if you only have 2 or 3 zones, but gets a bit harder (an aesthetically weirder) at 4+.

 

[_Jerry_]

Thanks.

Lots of info to digest, especially on solar. Quick takeaway is heat pump has about 30% higher operating cost than gas condensing boiler using the efficiency and model units you assumed. But I can save a bit on upfront installation cost with heat pump because its 2 units in 1. Adding solar may make up the difference, if not offset heating operating cost entirely. Its definitely something to consider in the future once I have everything setup.

A quick reply to confirm your understanding of my house layout to get an idea of suitable number of heads and head placement for the ductless option (whether is multi-split or several mini-splits). Let me know if you need photos of the house.

You are correct. Den and garage are on street level occupying half the building footprint. Half a level up (about 6’) is the living/dining room/kitchen covering the other half of the footprint. Then upper floor bedrooms overlying den and garage. Stairwells are open. Except there is a door for stairs from den to basement at the den level (can leave that door open if needed).

If I understand correctly that head placement for cooling needs is generally straightforward, but tricky for heating needs. For cooling, one head in general vicinity of upper floor stairway should serve all cooling needs of the entire house by leaving doors open. Or maybe add a small head at the living room or den level to cool the basement? I do not have any big west facing windows or sliding doors. May also be tricky leaving bedroom doors open with 2 toddlers - but will make it work if thats what it takes. What about head placement for heating needs? Does each room need a heat source? Very curious to what your recommendations are or your past experience on this.

Obviously I need to do a sufficient job on insulation upgrade. Also note that for short term, I do not plan on insulating foundation walls in basement because of time and cost (basement is finished). I can leave basement-den stairway close if it helps, and do not care if basement is cold this winter. Let me know how much this will impact heating needs and head placement.

Thanks,
Jerry

 

[_Jerry_]

A follow-up on gas unit cost.

$1.58/therm is based on Sept. 2016 billing (my first bill since moved in). I just found out the unit cost from Nov. 2015 thru March 2016 ranged from $0.91 to $1.20/therm. That makes it about $10.80/MMBTU vs $21/MMBTU for electric. I thought winter gas cost will be more expensive, apparently not the case.

Local plumbing supply company told me an HTP UFT-80w will cost about $1500. About $900 net if I include $600 rebate from local gas company, plus other parts and installation.

Thoughts?

 

[Dana]

Open doorways & staircases improve the prospects of being able to heat /cool efficiently with ductless systems. The heating & cooling loads of individual room are often well below the low end of the modulation range of a ductless head, and if it's cycling on/off rather than modulation most of the time, efficiency suffers. The very common but efficiency-robbing mistake made by many installers (often at the insistence of the homeowners) is to put a ductless head in every room, independent of the heating/cooling loads.

Natural gas pricing is very volatile, but the current gas glut (primarily to the recent innovations putting a lot fracked shale gas on the market, and a lack of export capacity for liquified natural gas) has been keeping prices low, much lower than they were 10 years ago. If more pipeline & LNG export infrastructure gets built that will lower prices in the shortest term, but increase pricing over the medium & long term.

A simple boiler swap to drop in the UFT-80w, along with decommissioning the existing AC, completely ripping out the ducts & air handler to allow proper air sealing & insulating of the attic, with a 1.25 or 1.5 heating/cooling mini split head at the top of the stairs is probably the right way to go. But an aggressive Manual-J load calculation on a room-by-room basis would be better than this WAG. Without knowing the orientation of the house and the amount of west facing window it's hard to say if the open living/dining area would need it's own AC. (Do the lower levels have cooling duct registers, or only the upstairs?) Worst-case, a 1.5 ton 2- head multi-split or a pair of smaller mini-splits would cover it, but you MUST do the load calculations to get it right.

If you can get away with just one head for cooling a 1.5 tonner should still be less than $4.5K (before any rebate subsidies), and will be far more efficient in cooling mode than any 2-3x oversized beast feeding leaky ducts in the attic.

Load calculations should not be left up to the HVAC company. Hire a properly credentialed engineer or RESNET rater to calculate what the loads will be AFTER the building envelope improvements, using aggressive rather than conservative assumptions on air leakage and R-values, etc. Be specific that you are looking for the most aggressive assumptions. With that information you can tell when the proposed equipment can work efficiently, and when it can't. Be sure to look at the minimum modulated cooling output relative to your 1% design load- if the minimum output is even half your design load, you won't get much modulating efficiency & comfort out of it.

With a heating/cooling mini-split you can probably cover more than half the heating load too, if/when gas prices spike, or when prices of solar eventually falls by more than half (which it will.) When it's over 40F outside the COP of a decent mini-split will be over 4.0 when modulating near it's minimum output, some will be over 4.5. This bench testing was on older models not quite as efficient as the best in class current versions. See figure 3, page 10.

Depending on the particulars it may be possible to cost-effectively insulate at least the above grade portion of the basement walls, which is where the bulk of the heat loss happens.

 

[_Jerry_]

Dana,

I understand you are proposing UFT-80w as primary heating, and mini-split heat pump as cooling and secondary heating. Question on energy consumption.

Say post-insulation heating load to be 30,000 BTU/hr for design temp range of 55F (between +70F and +15F). For average seasonal temp between 40F to 45F (temp range half of 55F), which is a cheaper heat source?

I believe the 11,000 BTU/kWh consumption rate for mini-split heating you provided above was at its rated heating capacity. For say the Mitsubishi MUZ-GE18NA 1.5-tonner, its rated heating capacity is 21,600 BTU/hr and power consumption at 1900 W - about 11,400 BTU/kWh. If I operate at its minimum heating capacity at 3500 BTU/hr during shoulder season, it consumes 230 W per its manual - that's 15,000 BTU/kWh or 35% more than its consumption at rated heating capacity. I am not sure how the condensing boiler consumption rate compares to mini-split during shoulder season. So I wonder if there is an optimal time to use mini-split heating where its cheaper than using condensing boiler.

Thanks,
Jerry

 

[Dana]

The GE18 with an HSPF of 10 (=10,000 BTU/kwh) is a lot LESS efficient than the FH18 with an HSPF of 12 (=12000 BTU/kwh) HSPF testing is the nominal efficiency when operated in heating zone IV (not to be confused with US climate zone 4, though you happen to be in both.)

That's fully 20% more heat per kwh going with the FH series, which makes a difference.

If your heat load is 30,000BTU/hr (but it isn't according to your calculations), for 55F delta, thats 30K/55F= 545 BTU/hr per degree F below 70F. The minimum modulated output is tested at +47F, which is a 23F delta, so ideally your mini-split would have a minimum modulation less than 23F x 545= 12,535 BTU/hr @ 47F so that it will be mostly modulating at part-load than cycling on/off at minimum load. Both the GE18 and FH18 would be fine. If you buttoned down the place and insulated to the point that the load was 20,000 BTU/hr you'd want a mini-splits that modulates down to below 8355 BTU/hr @ 47F. The higher 5,150 BTU/hr minimum output of the FH18 will not be a problem- it would meet or beat it's efficiency numbers in your application.

The "11,000 BTU/kWh consumption rate" isn't a consumption rate, its an energy efficiency rate. A kilowatt hour or BTU is not a rate, but fixed amount of energy, using two different setss of units, but are not time dependent. It means for every kwh the thing uses, it delivers an average of 11,000 BTUs, but tells you nothing about the rate at which those BTUs are being delivered. A coefficient of performance (COP) is energy-out/energy-in, but HSPF is a hybrid way to express the same thing, using watt-hours as the input energy unit, and BTU as the output units. The simple unit conversion of BTU to kwh energy units is 3412 BTU per kwh. An HSPF of 11 means it should average 11 BTU out for every watt hour in. Since electricity is metered and billed in units of 1000 watt hours (kwh), multiply by 1000, to get 11,000 BTU/kwh.

And the "rated" capacity isn't it's maximum capacity. It is the modulation level at which it was tested for HSPF efficiency. It's maximum capacity varies with temperature, and it's efficiency varies with both temperature and modulation level. The HSPF tests at a lower modulated output than the "rated" output at +17F than it does at 47F, and the performance data from both sets of temperature & "rated" capacity is fed into an mathematical model to estimate it's average performance in a zone IV climate. In the case of the GE18 it's rated capacity at +17F is an anemic 13,400 BTU/hr, but it's maximum @ +17F is only 17,200 BTU/hr,( which isn't even as high as they tested it's "rated" efficiency at +47F. Most models use the max output at +17F for the modulation level of testing at +47F, but here they probably chose numbers that would get them to an HSPF of 10.)

The FH18's internal controls inhibit it from delivering more than it's 47F "rated" 20,300 BTU/hr when it's +17F or cooler outside, forcing it to operate at part-load 20,300 BTU/hr at which it was tested at +47F., and higher efficiency, even though the same fans & coils operated differently could deliver higher capacity. Even at +5F it can still deliver 20,300 BTU/hr, but when it's colder, it may not, depending on outdoor humidity, etc.

The HSPF test was designed for single stage & 2 stage ducted heat pumps, and don't accurately reflect the potential for savings with fully modulated units. The COP of the FH18 at +47F is 3.46 even when delivering the full "rated" 20,300 BTU/hr, but at a modulated 8355BTU/hr or 12,535 BTU/hr it's COP efficiency will be over 4. The referenced bench testing document was done on older 1-tonners, but the characteristics of higher efficiency at part load is true for the 1.5 tonners as well. The "rated" level of output was about 2/3 the maximum at +47, but in your case you would be operating at about 1/3 the rated output, give or take, and probably operating in the COP 4.5-5 range with the FH,18, 4-4.5 range with the GE18 until it's warm enough that it's cycling a lot rather than modulating.

A COP of 4.5 means it delivers (4.5 x 3412= )15,354 BTU/kwh of power use or about 65 kwh/MMBTU. So at 21 cents/ kwh costs (65 x $0.21=) $13.65/MMBTU.

At a COP of 5 that would be 17,000 BTU/kwh, or 59 kwh/MMBTU, which would cost $12.39/MMBTU, which is still more expensive than condensing gas. But it's closer than the raw numbers suggest, since the power used by the boiler & pumps are not being accounted for.